Abstract
Continuous measurements of ozone and its precursors were performed at sites in two Chinese megacities, i.e., an urban site in Beijing and a suburban site in the Pearl River Delta (PRD). At both sites, the total oxidants (O3 + NO2) varied with the ratio of ethylbenzene to m,p-xylenes, which serves as an indicator of photochemical aging. An observation-based method (OBM) was derived for calculating the photochemical consumption of individual non-methane hydrocarbons (NMHCs) based on the observed NMHC concentrations and the ratio of ethylbenzene to m,p-xylenes. The results show a strong correlation between the oxidant level and the derived consumption of precursors at the two sites (R2 = 0.81 for the PRD site and R2 = 0.83 for the Beijing site), demonstrating a strong cause–effect relationship. The relative “consumption efficiency” among NMHCs was calculated based on the integrated amount of hydroxyl radicals derived from the ratio of ethylbenzene to xylenes. Thus, the percent contributions to ozone formation from each individual NMHC can be calculated. This concept of consumption is purely observation-based and provides an easy way to bypass complicated modeling and the necessity of knowing instantaneous concentrations of hydroxyl radicals, which are highly illusive in nature.
Highlights
Beijing, the capital of China, is a major urban center located on the North China Plain
It is well known that ozone in the lower troposphere is directly linked to its precursors—volatile organic compounds (VOCs) and NOX (e.g., [9,10,11])
The time series of oxidant (O3 + NO2 ) concentrations and E/X ratios at both sites are shown in Figure 1, which illustrates the diurnal variation of photochemical processing
Summary
The capital of China, is a major urban center located on the North China Plain. Over the last few decades, rapid economic growth and an increased number of vehicles in Beijing increased its atmospheric pollution. 2013, PM2.5 levels dropped significantly, but ozone pollution did not improve [4,5]. The ozone levels in Beijing frequently exceeded the national daily ozone standard (0.20 mg/m3 , approximately 100 ppb), with more occurrences in the summer than in other seasons [6]. The rapid economic growth was accompanied by worsening air quality, with ozone being one of the most important pollutants [4,7,8]. Most investigations of the relationship between ozone and its precursors are based on models that rely on inputs of precursor emission rates, chemical reactions and atmospheric dynamics. Model-based predictions of ozone concentrations are dependent on assumptions
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